The whys and wherefores of gastrulation

Abstract

Gastrulation is defined as the phase of morphogenetic movements lying between the blastula stage and the phylotypic (zootype) stage of development. The occurrence of gastrulation is universal among animals undergoing embryonic development from eggs, but there is considerable diversity in the types of movement observed. Gastrulation is also the stage at which the first very important developmental commitments are established, in particular those corresponding to the classical 'germ layers'. From an evolutionary point of view, animal development falls into three phases of which the first and last are variable and the middle, phylotypic, phase is most conservative. Gastrulation belongs to the early phase of development which is inherently variable because it is subject to selective forces operating on reproductive behaviour and life history, principally those controlling the number and size of eggs, and the means for embryonic nutrition.